Amplitude control circuits (ACC's) control the power provided and processed by systems. Also, ACC's can allow systems to adapt to changeable operating environments and signal dynamics. ACC's can be used in any suitable system, such as a communications system, a process control system, test equipment, and an audio control system. For example, in a communications system, amplitude control at a receiver ensures a constant amplitude level is provided to a demodulator. Also, amplitude control at a transmitter aids in reducing the dynamic range requirements of the receiver.
Many systems, such as communication systems and audio control systems, include at least one voltage controlled oscillator (VCO). A VCO provides a selected oscillating frequency that can be adjusted with a control signal. Typically, the control signal alters a component value, such as a capacitance value, to change the oscillation frequency. An ACC coupled to a VCO controls the amplitude of the oscillating output signals. VCO's can be used in any suitable system, such as a portable communications system.
Typically, VCO's in portable communication systems are built to provide low phase-noise levels while consuming minimal power. To achieve these goals, manufacturers have developed VCO's in complementary metal oxide semiconductor (CMOS) technology. In one CMOS VCO including an ACC, the ACC provides current to the VCO without feedback from the differential output signals of the VCO. The results are process dependent. Also, bipolar junction transistor VCO's including an ACC do not transfer well to CMOS technology and further, to low voltage CMOS technology.
For these and other reasons there is a need for the present invention.